1. Field of the Invention
The present invention relates to a plain light source device, and more particularly to a plain light source device having a commonly used fluorescent tube as a light source for a backlighting of a light transmission type liquid crystal display device.
2. Description of the Prior Art
When a fluorescent tube is used as a light source in a plain light source device for a backlighting of a light transmission type liquid crystal display device, there arises the problem that the fluorescent tube generates noise because of the application of a high voltage, which adversely affects the liquid crystal display device.
To avoid this problem, there have heretofore been taken such measures as providing a reflector formed from a metal or a metal evaporated resin (plastic) as shown in FIG. 1. In FIG. 1, the numeral 1 indicates a resinous (plastic) reflector, and 2 a metal evaporated layer.
Also, to prevent the noise interference to the display panel (liquid crystal panel), there have been taken such measures as disposing a mesh-like metal plate 3 shown in FIG. 2 above the fluorescent tube or providing a transparent conductive sheet on the rear side of the panel.
Generally, light transmission type liquid crystal display devices require a high-luminous uniform backlight, that is, a plain light source device.
However, when the reflector formed from a metal or a metal evaporated plastic, as shown in FIG. 1, is used, the luminance distribution on the display screen greatly varies depending on the shape of the reflector. This makes it extremely difficult to design a proper reflector, and a separate device such as a light curtain to be disposed above the fluorescent tube is required for uniform luminance distribution.
Also, when the mesh-like metal plate 3 is provided above the fluorescent tube, as shown in FIG. 2, to prevent the noise interference to the liquid crystal panel, the light emitted from the fluorescent tube is blocked by the metal plate 3. This reduces the efficiency of light utilization and therefore requires an increase in the output of the fluorescent tube to obtain high luminance, which results in a decrease in the efficiency of the lighting circuit.
Furthermore, when the transparent conductive sheet is mounted on the rear side of the display panel, a complicated grounding method is required.